1. Field of the Invention
The present invention relates generally to ultrasonic cleaning, and more specifically, to a method and apparatus for cleaning deposits from flow nozzles or venturis by causing ultrasonic cavitation in a fluid flowing therethrough.
2. Description of the Related Art
Nuclear power plants usually operate at the full thermal power for which they are licensed. The percent of thermal power at which the unit is operating is determined by the final feed water flow times the enthalpy difference between the steam generator output and inlet times a conversion factor times 100 divided by the licensed power. The resulting value may not exceed 100%. The closer to 100% thermal power the unit operates, the more megawatts are produced by the generator which are available to the utility's customers. Anything which limits achieving 100% thermal power results in increased costs for the utility because the missing megawatts must be replaced either by increasing the megawatts produced by one of their higher-operating-priced fossil units, or by purchasing the megawatts from another utility at a premium. For example, the losses associated with operating a 1,000 megawatt nuclear unit at 98% thermal power instead of 100% will be approximately 20 megawatts. This translates into an extra cost to the utility of as such as $10,000 per day to replace the megawatts with another unit, or a minimum of $30,000 per day to purchase replacement megawatt-hours.
A common cause of low thermal power is fouling of the feedwater flow nozzles or venturis. This has been a troublesome problem to the nuclear power industry. The nozzles or venturis which operate based on the Bernoulli flow theorem are used to measure the feedwater flow. Water flowing in the nozzle or venturis is accelerated through a reduction in area, and the increase in water velocity causes a corresponding decrease in pressure. The flow is proportional to the difference in pressure measured at the inlet to the nozzle (the large area) and the pressure measured at the throat of the nozzle (the small area). As the nozzle fouls, deposits adhere to the surface of the nozzle, thus decreasing the throat area. The true flow (which remains unchanged) is now being forced through a greater reduction in area, and this results in a greater difference in pressure. The higher pressure differential causes a higher flow to be calculated. This higher calculated flow is fictitious because of the effect of the deposits. The real flow remains the same. When the calculated feedwater flow rate is used in the thermal power equation, it results in a fictitiously high thermal power calculation. As the nozzles foul, the utility makes adjustments to maintain full thermal power. The utility is led to believe that the unit is operating at 100% thermal power when in reality it is not. The outcome is that the unit is producing fewer megawatts than it could, and the utility ends up paying additional cost to replace the missing megawatts.
Feedwater nozzle fouling can be reduced by paying close attention to steam/water chemical purity in the power plant, and by eliminating known sources of deposits (such as, copper tubes in heaters or condensers). Not every chemical upset can be prevented, however, and tube replacement is an expensive solution which requires an outage. Even so, fouling cannot be totally prevented.
Other means to circumvent this problem have been developed. One such development is the use of a port in the nozzle assembly through which a water jet can be inserted to wash away the deposits. This solution, however, requires an outage. Another development is the use of a type of flow measuring device which is not effected by deposits, such as the leading edge flowmeter which is based on the Doppler Effect. However, this device is expensive and requires special maintenance.
Ultrasonic cleaning systems have been developed in recent years. Ultrasonic cleaning is a non-destructive method which uses sound waves to cause cavitation in a liquid. Cavitation is the formation and implosion of tiny vapor pockets which release energy in the form of instantaneous high pressures and temperatures. The cavitation creates a scrubbing action which loosens and removes contaminants while leaving the surface entirely unaffected.
A device which uses ultrasonic transducers to clean deposits from flow, nozzles is described in U.S. Pat. No. 4,762,668, issued on Aug. 9, 1988. The patent describes an ultrasonic cleaning device for a venturi flow nozzle mounted in a pipe in a fluid system in which a transducer is mounted adjacent the pipe for the purpose of producing sound waves. A rod is connected at one end to the transducer and extends through an opening into the pipe so that the other end of the rod contacts the nozzle to transmit the sound waves to the nozzle. A guiding and sealing assembly is also provided for the rod for attachment to the pipe around the pipe opening. In operation, the cleaning device is mounted to a pipe and venturi nozzle, or each transducer assembly is supported solely by each guiding and sealing assembly. Power is supplied to the transducers by any suitable source through electrical connections. During use, an automatic timer is employed to actuate the transducers as desired. The sound waves generated by the transducers prevent fouling in the venturi flow nozzle by preventing deposits from building up. The sound waves, which create ultrasonic cavitation, are designed to excite the nozzle directly.
A continuing need exist for improved methods and apparatuses which are capable of easily cleaning venturi flow nozzles, and which are relatively inexpensive and easy to install and which do not require plant shut down and pipe drainage to implement.